Many premises
installations are
now using 50/125 fiber for Gigabit and 10 Gigabit Ethernet instead
of 62.5/125 fiber, since the new laser-optimized 50/125 fiber
is a better solution for high speed systems with laser sources
and still compatible with virtually every system that will run
on 62.5//125 fiber. Since little 50/125 has been used in the last
decade in the USA other than by the US government and military
(it's also used in Europe and Japan), it represents a major departure
from the standard fiber of the past.

One issue
created by the resurgance
of 50/125 fiber is the complications of mixing the two fibers
in one installation. Mixing the two, as can happen with installations
that have both and need both types of patchcords, can induce severe
losses in links. New TIA standards call for color coding cables
and patch panels to prevent mixups. See
this Tech Topic for more detail.

If one
connects a 50/125 fiber
to a 62.5/125 fiber, the smaller core of the 50/125 fiber easily
couples to the 62.5/125 fiber and is very insensitive to offset
and angular misalignment. However, the larger core of 62.5/125
fiber overfills the core of the 50/125 fiber, creating excess
loss.

The
traditional range of mismatch coupling losses has been covered in
several documents, including The Fiber Optic
Technicians Manual, Table 17-4 (Excess loss in dB) covering
the full range of legacy multimode fibers:

Transmitting Fiber

Receiving
Fiber

62.5/125

85/125

100/140

50/125

0.9-1.6
dB

3.0-4.6
dB

4.7-9.0
dB

62.5/125

-

0.9
dB

2.1-4.1
dB

85/125

-

-

0.9-1.4
dB

There has
been much speculation
as to what the power penalty would be, especially when the system
uses a GBE VCSEL source with it's narrower modal fill in the fiber,
rather than a LED used for slower systems and most test equipment.
As part of tests of cable plant loss with LEDs and VCSELs, we
decided to see for ourselves the magnitude of the power penalty.

We set up a
test to find the excess
loss or power penalty that can occur when mixing 50/125 and 62.5/125
micron multimode fibers in a single link. To make the test more
relevant for current networks, we used both a standard 850 nm
LED source and a 850 nm VCSEL for test sources. The tests were
done for both three different conditions that should have relevance
to all normal circumstances.

Test
1:
Launch with 1 meter 62.5/125 cable, as with a
source with 62.5/125 patchcord connected to a network cable plant
of 50/125 fiber.

Test
2:
Launch with 20 meter 62.5/125 cable with one connector
in the middle, such as the receiver would see in a short floor-to-floor
link and a 50//125 patchcord at the receiver.

Test
3:
Launch with 520 meter 62.5/125 cable with three
connectors in the middle, the longest normally expected link for
GBE into a 50//125 patchcord at the receiver.

In all three
cases, the receive
cable was a 1 m 50/125 jumper, to minimize all other loss factors,
such as fiber losses.

Fiber excess loss connecting 62.5 to 50
micron

.

LED

VCSEL

LED

VCSEL

LED

VCSEL

.

Test 1

Test 2

Test 3

Launch
from

1m 62.5

20m 62.5

520m
62.5

Control
62.5

0.33

Not
Tested

0.12

0.05

0.05

0.04

1A

3.39

1.17

2.30

1.05

1.97

1.16

1B

3.39

1.26

2.33

1.07

1.98

1.13

2A

3.36

1.29

2.39

1.08

1.92

1.13

2B

3.37

1.33

2.31

1.00

2.46

1.14

3A

3.70

1.29

2.31

1.11

2.06

1.15

3B

3.37

1.29

2.39

1.03

2.02

1.07

4A

3.40

1.37

2.58

0.98

1.94

1.14

4B

3.49

1.26

2.36

1.09

1.97

1.10

Average

3.43

1.28

2.37

1.05

2.04

1.13

Std.
Dev.

0.11

0.05

0.09

0.04

0.16

0.03

The data
follow predictions very
well. Losses are higher with LEDs than VCSELs, showing the lower
mode fill of the VCSEL. LED losses go down as we go to longer
launches, indicating the LED mode fill decreases. Note, however,
the VCSEL has slightly lower excess loss at 20 meters than at
either 1 meter or 520 meters. This is similar to data we obtained
in our earlier tests, where the mode mixing from the extra connectors
in longer cable plants makes the VCSEL differences level out in
only short cables.

The data is
very definitive, however.
The excess loss caused by coupling 50/125 fiber to 652.5/125 fiber
enacts a severe penalty on power budget, especially when for used
GBE networks with their lower power margins.

Mixing 50/125
and 62.5/125 fiber
in one cable run is not sensible. If one is using 50/125 fiber
instead of 62.5/125, all components - cable and patchcords should
be 50/125. The best way to segregate 50/125 and 62.5/125 fiber
in one installation is to color
code the cables to keep them separate and/or use different
connectors. Since the LC connector is becoming the de facto standard
for gigabit and above, using LCs for 50/125 fiber is a bulletproof
solution.

Editorial
comment: This is
a technical analysis, not an editorial, and should under no
circumstances
be be interpreted as a reason that one should not upgrade to
50/125 laser-rated fiber! This analysis was done simply to update
data from two decades ago to include VCSELs which became available
with Gigabit Ethernet. In fact, we suggest using 50/125 fiber
for the advantages afforded by the higher-bandwidth 50/125 fiber.
The no-mixing issues are no different than UTP Cat 3 and Cat 6!